Digital telecommunication transmission networks are normally composed of two types of network elements. A first element is the terminal which terminates the end of a transmission span and distributes the traffic to central office switches and other signal processing equipment. A second element is a regenerator which simply regenerates and retransmits the transported signal at mid span points. A repeater typically comprises two regenerators, normally used for bidirectional traffic paths. Often, the terminal comprises time division multiplex/demultiplex (TDM) equipment as well as the transmit and receive functions. The combination of several traffic channels into one transport channel allows more economical use of the transport media than with separate allocation of transport channels to traffic channels.
A third element has been introduced into network configurations in recent years. This third element is known to those skilled in the art as an add/drop muldem. The add/drop muldem provides the same functionality as back-to-back terminals. In the past, this functionality was achieved through the physical connection of the office input and output connections of one terminal network element to another within the same office. This approach required that each unit be fully equipped with I/O units and that inner office cabling be provided to make the connections. The add/drop muldem element simply combined the muldems, transmitters and receivers in one functional package to eliminate the need for extraneous I/O units and support equipment. Thus, traffic channels whose presence was not required at a location could be passed through to the outgoing traffic with a minimum of processing.
There are two fundamental types of digital telecommunications transmission networks that can be built from the basic network elements discussed supra. These are the linear network and the ring network. The ring network can be built in either a unidirectional or bidirectional format. The unidirectional ring utilizes the add/drop muldem as the base network element. Such a structure utilizes two fibers or transport paths for transmission of traffic between the sites.
It is possible to combine the two types of networks into a single higher transport capacity network to economize on transmission paths usage between two locations. In such a situation, one or more I/O ports may serve as the transmission paths of a unidirectional ring while other I/O ports serve linear networks. The high capacity transport muldems multiplex the various lower rate I/O's into one or more high capacity transport signals.
Protection of the transmission networks can be accomplished in several ways. Linear systems are usually protected through the use of 1:N systems. Ring networks are normally protected on a 1:1 basis using line protected switching or path protected switching on either unidirectional or bidirectional rings. Recent introduction of digital cross-connect systems have introduced the mesh concept of protection switching. Protection against failure of the transport system network elements or interconnecting paths is often provided by 1:N protection switches where N is .gtoreq.1 and generally is less than 14. Switching is normally done at the common rate I/O interface to divert traffic from a working system I/O to a dedicated protection channel I/O. A straightforward 1:N switching system may be built with linear network through-put traffic from tributary shelves. Working channels may be designated 1, 2, . . . N and the protection channel may be designated P. When the switches are activated, the I/O traffic on the common rate connection into the high capacity transport system is diverted to channel P. Activation of the switches comes from the working channels directly or through protection channels. The switches are normally coordinated on a end-to-end span basis to prevent unnecessary loss of traffic during switching and restoring processes. This type of protection switching system will protect against the loss of traffic from the failure of any one working element or transport path. Failures such as optical fiber cuts normally break all the paths within a site-to-site connection and therefore, cannot be protected through such a network. One of the major disadvantages of this last mentioned network is that the protection channel P is idle most of the time. There are ways in which the previously referenced system can be reconnected to carry low priority traffic during times when it is not needed for protection of a working channel. When a working channel fails, I/O traffic is switched from the protect channel access tributary I/O to the working channel I/O. Control of the switches would be similar to that mentioned previously.
In communication systems where there are diverse rate I/O requirements, additional multiplex/demultiplex devices are added to bring all signals fed to the protection switch to a common rate or format suitable for connection to the protection switches and I/O ports of the transmission equipment. These multiplex/demultiplex devices are called tributary shelves herein.
Additional protection means is commonly placed in the tributary shelves to protect the traffic against failures in the shelves. Most often, these shelves will incorporate 1:1 protection (i.e. main unit, spare unit and a 1:1 switch at the common rate ports of the shelf). Other units within the shelf may be protected in many different ways.
The basic concept of the present invention combines a ring network concept along with tributary shelf protection where the tributary shelf muldens are designed to be add/drop muldems in lieu of the 1:1 protected straight I/O muldems. The add/drop muldems are switched to a LOOPed condition of the high speed I/O terminals to form the ring whose transmission paths connect to the low speed I/O of the high speed transport equipment. The advantage of this configuration over the standard approach is that it is generally less expensive to incorporate the LOOPed/THRU switching capability of the add/drop muldem into the tributary shelf muldems than it is to provide the stand alone 1:N switch hardware/software and tributary shelf muldem protection switching of the referenced prior art. The use of the add/drop muldem permits 1:N switching to be employed on ring networks embedded in a high capacity transmission system. The protection channel can be used in a similar manner to the prior art for low priority traffic or non-critical traffic by installing tributary shelves in the protect channel ring and setting the add/drop muldems to operate normally in the open or THRU mode. When a switch request is made for a working channel to switch to the protect channel ring, these muldems revert to the LOOPed mode described earlier.
Combining the feature of ring transport through a linear high capacity transport with the ring protect 1:N feature permits protection of traffic distributed throughout a campus, building or other environment. Since I/O paths through each of the add/drop muldems can be looped or add/dropped, the composite rate signal from the tributary shelves can be made up of tributary I/O from several shelves as in a unidirectional ring. Protection switching can be accomplished in the same manner as described above with all of the tributary shelves on the ring acting as one. The addition of optical interfaces or electrical line drivers and receivers to the add/drop muldems allows the tributary shelves to be remotely located with almost no penalty. The add/drop muldems can function as regenerators when in the LOOPed mode and thus, reenforce the signal to the next shelf. In addition, the internal workings of these looped muldems can provide performance monitoring and fault location capabilities for the protect ring.
It is therefore an object of the present invention to provide an improved protection mode configuration of a plurality of communication networks having auxiliary units capable of normal and LOOPed configurations similar to that of an add/drop muldem.